PROJECT SUMMARY/ABSTRACT Recent studies have implicated the superior colliculus (SC) and parabigeminal nucleus (PBG) as important structures involved in the selection and activation of visually-evoked defensive behaviors, such as freeze and escape. However, the precise role that the SC and PBG play in the initiation of visually-evoked behavior is unknown, in part due to a gap in knowledge of the synaptic circuits that link the SC and PBG, and the manner in which the PBG receives visual signals. The proposed project will address this gap by completing a detailed examination of circuits that link the SC to the PBG. Aim 1 experiments will define the morphology and laminar distribution of SC cells that contain glutamic acid decarboxylase (GAD) or parvalbumin (PV) and send projections to the PBG. Aim 2 experiments will test whether these two cells types receive direct or indirect input from the retina and/or primary visual cortex. Aim 3 experiments will define the synaptic properties of SC-PBG connections. To investigate these circuits, the proposed experiments use novel combinations of viral vector injections in transgenic mice, immunohistochemistry, in vitro whole cell recordings from identified cell types, and optogenetic activation of cell-type specific synaptic terminals. This combination of approaches will address fundamental questions regarding the circuity of the SC and PBG while providing the applicant with broad-based training in neuroscience.